1. Field of the Invention:
The present invention generally relates to a high-voltage interface. In particular, it relates to a photocoupler that withstands a high voltage used for home electric appliances.
2. Description of the Related Art:
One of the most popular home electric appliances may be an air conditioner. Referring to FIG. 1, typically an air conditioning system includes an internal unit 1 and an external unit 2. Though not depicted, the external unit 2 may include a compressor, a fan, and other necessary components. The driving power for the external unit 2 is supplied from the internal unit 1 via a power cable 3. As illustrated, the power cable 3 includes a pair of conductive lines 3a and 3b which are insulated from each other. An alternating current of 100V may be carried by the power cable 3.
In addition to the power cable 3, a single conductive line 4 is used between the internal unit 1 and the external unit 2. The conductive line 4 transmits control signals to manage the operation of the external unit 2. For economy of the system, the power line 3b is also used for transmitting the control signals (thus, the power line 3b works as both power transmitter and control signal transmitter).
To properly operate the movable components in the external unit 2 (i.e. the compressor, the fan, etc.), the control signals supplied to the unit 2 should be free from electrical noise. To achieve this, use may be made of an interface (I/F) such as a photocoupler. The interface provided in the internal unit 1 is connected to the power line 3b and the control signal line 4.
FIG. 2 shows the principal components used in a conventional photocoupler PC1xe2x80x2. In the photocoupler PC1xe2x80x2, a photodiode PD1xe2x80x2 is provided on the primary side, while a phototransistor PT1xe2x80x2 and a transistor TR1xe2x80x2 are provided on the secondary side. The collectors of the respective transistors PT1xe2x80x2 and TR1xe2x80x2 are connected to a first output terminal, while the emitters of the respective transistors PT1xe2x80x2 and TR1xe2x80x2 are connected to a second output terminal. The first output terminal is connected to the power line 3b (FIG. 1), while the second output terminal to the control signal line 4.
In operation, an unduly high voltage may be applied to the two transistors PT1xe2x80x2 and TR1xe2x80x2 via the power line 3b. Thus, as aprecaution, the transistors PT1xe2x80x2 and TR1xe2x80x2 need to be designed as a high voltage device. In general, however, a high-voltage transistor tends to exhibit poor switching characteristics (i.e. slow response) due to the collector-base junction capacitance. This is disadvantageous because the interface using the high-voltage phototransistor PT1xe2x80x2 may fail to transmit the control signals properly.
The above problem may be overcome by providing an electrical connection between the base of the phototransistor PT1xe2x80x2 and the output terminal 2. With such a connection, the unfavorable charge appearing between the collector and the base of the phototransistor PT1xe2x80x2 will be eliminated.
The above scheme, however, will cause another problem. Specifically, for making a connection between the transistor PT1xe2x80x2 and the output terminal 2, an additional step is required. Due to this, the circuit layout of FIG. 3 will cost more than that of FIG. 2. Accordingly, the product photocoupler adopting the layout of FIG. 3 becomes expensive.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an interface which has quick response and high withstand voltage, and which can be produced at a low cost.
According to the present invention, there is provided an interface which includes: a signal relay unit that transmits a signal from a primary side to a secondary side; and a voltage regulation unit that applies to the signal relay unit a voltage which is lower than a predetermined threshold, wherein the voltage regulation unit has a greater withstand voltage than the signal relay unit has.
Preferably, the signal relay unit includes a photodiode that emits light and a phototransistor that detects said light.
Preferably, the signal relay unit includes a protection package that encloses the photodiode and the phototransistor.
Preferably, the voltage regulation unit includes a first high-voltage transistor and a zener diode, wherein the first high-voltage transistor has a higher withstand voltage than the phototransistor has, and wherein the zener diode is connected in parallel to the phototransistor.
Preferably, the interface of the present invention further includes a signal amplification unit that amplifies a signal supplied from the signal relay unit.
Preferably, the signal amplification unit includes a second high-voltage transistor connected to the phototransistor of the signal relay unit, wherein the second high-voltage transistor has a greater withstand voltage than the phototransistor has.
Preferably, the phototransistor and the second high-voltage transistor provide, in combination, a Darlington transistor.
Preferably, the signal amplification unit includes a resistor which is disposed between the base and the emitter of the second high-voltage transistor.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.